JpGraph supports both a set of built in bit-mapped fonts as well as True Type Fonts. For scale values on axis it is strongly recommended that you just use the built in bitmap fonts for the simple reason that they are, for most people, easier to read (they are also quicker to render). Try to use TTF only for headlines and perhaps the title for a graph and it's axis. By default the TTF will be drawn with anti-aliasing turned on.
In many of the example you can see examples of both TrueType and Bitmap fonts.
There are a number of subtle differences in the way builtin fonts and TrueType fonts are rendered to the screen. However, JpGraph, abstracts away 99.9% of the differences so it will be, for the user, completely transparent to switch between the different fonts.